Hexagonal boron nitride (hBN) is also referred to as boron nitride, which is of a layered structure similar to graphite. With the advantages of good electrical insulating property, low dielectric constant and dielectric loss, high-temperature stability, good lubricity and chemical inertness and metal nonwetting property, hexagonal boron nitride is widely used as a solid lubricant in the high-temperature environment, a stripping agent in cast molding and injection molding, an evaporator boat for vacuum aluminizing, a wave-transmitting material, etc.
Hexagonal boron nitride ceramics are generally manufactured by causing powder which is formed after nitriding or ammonia decomposition of boron trichloride to undergo ball-milling mixing with boron oxide agglutinant and undergo pressureless sintering, hot pressing, thermal decomposition or a combustion technology. However, as the bonding force of hBN in the C-axis direction is far smaller than that in the direction perpendicular to the C-axis direction, crystal mainly grows in the plate surface direction, the growth in the thickness direction is slow, and thus a tabular crystal structure is formed. The tabular crystal structure forms a clamping piece bridging structure during sintering to achieve the effect of mutual supporting and hinder shrinkage of the material, and thus the obtained hBN ceramics are low in density. For example, after Hagio et al adopted a ball-milling method to perform mechanochemical activation on hBN powder, the density of the hBN ceramic material obtained at a sintering temperature of 2000° C. was 1.64 g·cm−3 and was only 70% of the theoretical density (Journal of the American Ceramic Society, 72(8) 1482-1484 (1989)). Kurita et al adopted AlN and amorphous B as additives and obtained an hBN material with a relative density of 75.8% by pressureless sintering in the atmosphere of N2 at 1500° C. (Shigen-to-Sozai; 105(2) 201-204 (1989)).
It is an effective measure to solve the hBN ceramic density problem by adding oxides such as B2O3, Al2O3, Y2O3 and SiO2 to serve as sintering additives and improving diffusion coefficient and sintering power during sintering through the liquid phase generated during the sintering. Particularly, SiO2 not only can promote sintering and densification of hBN, but also can improve oxidation resistance of hBN and usage temperature under high temperature, and thus SiO2 receives extensive attention. Chen et al prepared hBN ceramics by combustion in the atmosphere of high-pressure nitrogen, and studied the influence of SiO2 on hBN density. It is found according to research results that the density of hBN without adding SiO2 is 71-75% while the density of hBN is improved to 75.4-78% after SiO2 with a weight percentage of 10 wt. % is added, which indicates that the adding of SiO2 improves the density of hBN effectively. On the other hand, since it is difficult to evenly disperse SiO2 in ball-milling material mixing, the relative density is still low (lower than 80%) (Journal of Materials Science Letters, 19 (2000) 81-83). In their disclosed patent (a Chinese invention patent with a publication number of CN1310149A), Han Jie-cai et al prepared hBN ceramics by adopting a combustion synthesis process. After being pre-pressed into blank, the reactive raw materials undergo self-propagating combustion reaction under the N2 pressure of no smaller than 70 MPa, and SiO2 powder with a weight ratio of no larger than 60% was added, so as to synthesize an hBN-SiO2 composite material. However, similarly, due to the fact that the raw materials were not mixed evenly, the density of the composite material was still low.